Energy absorbing material

ABSTRACT

There is provided a self-supporting energy absorbing composite comprising: i) a solid foamed synthetic polymer matrix; ii) a polymer-based dilatant, different from i) distributed through the matrix and incorporated therein during manufacture of i); and iii) a fluid distributed through the matrix, the combination of matrix, dilatant and fluid being Such that the composite is resiliently compressible. There is also provided a self-supporting energy absorbing composite comprising: iv) a solid, closed cell foam matrix; v) a polymer-based dilatant, different from i), distributed through the matrix; and vi) a fluid distributed through the matrix, the combination of matrix, dilatant and fluid being such that the composite is resiliently compressible.

This invention relates to energy absorbing materials e.g. of the kindemployed in systems designed for the protection of humans, animals orobjects from damage by impact; referred to hereinafter as impactprotection systems.

Conventionally, impact protection systems have employed, as the energyabsorbing material, elastomer foams or similar relatively softresiliently compressible materials. However only limited protection isachieved. In some systems, this energy absorbing material is employed incombination with a rigid member the purpose of which is to spread theimpact force over a greater area and therefore reduce its effect.However, such systems tend to be inflexible and uncomfortable if incontact with a human body. Most vulnerable areas of the body whichrequire protection, e.g. elbows and knees, undergo significant changesin geometry and thus any attempt to match a rigid load-spreading shapewill usually fail. One solution is to introduce articulation into therigid element but this can compromise performance and increases cost.

More recently, proposals have been made for the use of shear thickeningsilicone putty materials, known as silicone dilatants, in or as energyabsorbing materials in impact absorption systems. By a shear thickeningmaterial or dilatant, we mean a material which viscously flows at lowrates of deformation but, at an elevated rate of deformation, undergoesa substantial increase in viscosity with rate of change deformation. Atsignificantly higher deformation rates, such as those induced by impact,the material becomes substantially stiff or rigid. For example, U.S.Pat. No. 5,599,290 describes a bone fracture prevention garment whichemploys, as the dilatant or shear-thickening material, a dispersion ofsolid particles in a viscous fluid. GB-A-2349798 describes an energyabsorbing pad including a putty-like dilatant. However, in both cases,the dilatant has to be contained in an envelope because of its nonself-supporting nature. The resulting products therefore tend to lackflexibility and will require relatively complex and expensivemanufacturing processes.

JP 6-220242 discloses a shock absorbing material which consists of aflexible, three-dimensional mesh or foam body which has interconnectedhollow spaces in its interior, and whose surface is coated with siliconebouncing putty.

The present invention provides an energy absorbing material suitable foruse in or as an impact absorption system and which is self-supporting.

According to one aspect of the present invention, there is provided aself-supporting energy absorbing composite comprising:

-   -   i) a solid foamed synthetic polymer, suitably an elastic,        preferably an elastomeric matrix;    -   ii) a polymer-based dilatant, different from i), distributed        through the matrix and incorporated therein during manufacture        of i); and    -   iii) a fluid distributed through the matrix, the combination of        matrix, dilatant and fluid being such that the composite is        resiliently compressible, and preferably also flexible.

By resiliently compressible we mean a resistance to compressive set.

By a solid matrix, we mean a matrix material which retains its ownboundaries without need of a container. Usually, the matrix will beelastic.

According to a second aspect of this invention, there is provided aself-supporting energy absorbing composite comprising:

-   -   i) a solid, closed cell foam matrix;    -   ii) a polymer-based dilatant, different from i), distributed        through the matrix; and    -   iii) a fluid distributed through the matrix, the combination of        matrix, dilatant and fluid being such that the composite is        resiliently compressible.

In addition to being self-supporting, the composite of the inventionoffers a degree of impact protection which can potentially exceed thatof current rigid systems and moreover, in the preferred embodimentwherein it is both flexible and resiliently compressible, it has theability to conform to the geometry of what it is designed to protect bymaintaining intimate contact through relatively large changes ingeometry. This is a key feature for the design of protective componentsbecause induced damage is a function of the maximum force resulting fromthe impact divided by the area over which this force is distributed. Thecomposite of the invention enables both a reduction in the force and anincrease in the area on which the force acts or is reacted, therebysignificantly reducing the resulting pressure or stress transmitted fora given impact energy. It also offers the ability to exhibit someconformity to the impactor and thus produce additional force absorptionas well as favourable geometry in terms of abrasion resistance. By meansof the invention, it is also possible to achieve improved performancecompared to the use of an equivalent mass of dilatant when used on itsown.

While it is to be understood that other solid materials may be suitablefor use as the matrix, in one, preferred embodiment of the invention,the matrix is selected from elastomers. While natural elastomers, e.g.latex rubbers, may also be used, our preference is for syntheticelastomers, including synthetic thermoplastic elastomers. One preferredclass of synthetic elastomers is elastomeric polyurethanes but it isexpected that others such as silicone rubbers and EP rubbers, e.g. EPDMrubbers may also be suitable.

In general, the resilient compressibility of the composite will beprovided by the fluid which is dispersed throughout the matrix. Usually,the fluid will be substantially uniformly dispersed throughout thematrix but non-uniform dispersion may be desirable in certain cases. Theresilient compressibility may be due to redistribution of fluid withinthe matrix and/or (in the preferred case wherein the fluid comprisesgas) compression of the fluid. Thus, for example, the combination ofmatrix and fluid may advantageously be a foamed elastomer, e.g., afoamed polyurethane elastomer, the foam may be open-cell, closed-cell orpart open, part closed. An important property of the foam is the rate atwhich it recovers after being subjected to compression. Preferably,recovery is complete or substantially complete within a few seconds,e.g. 5 seconds or less, more preferably 2 seconds or less. However, aslower rate of return may actually be preferably for some applications.

Any polymer-based dilatant that can be incorporated into the chosenmatrix may be used. By a “polymer-based dilatant”it is meant a materialin which the dilatancy is provided by polymer alone or by a combinationof polymer together with one or more other components, e.g. finelydivided particulate material, viscous fluid, plasticiser, extender ormixtures thereof, and wherein the polymer is the principal component. Inone preferred embodiment, the dilatant is selected from siliconepolymer-based materials exhibiting dilatant characteristics. Thesilicone polymer is preferably selected from borated silicone polymers.The dilatant may be combined with other components in addition to thecomponents providing the dilatancy, e.g. fillers, plasticisers,colorants, lubricants and thinners. The fillers may be particulate(including microspheres) or fibrous or a mixture of particulate andfibrous. One class of particularly preferred dilatants comprises theborated siloxane-based material marketed by Dow Corning under catalogueno. 3179 where polyborondimethylsiloxane (PBDMS) constitutes the basepolymer.

Other polymer-based dilatant materials having similar dilatancycharacteristics, e.g. a similar modulus at low rates of strain and asimilar plot of modulus against strain rate are also included.

The composite of the invention may be formed by combining a solidmatrix, a polymer-based dilatant and a fluid whereby the dilatant andfluid are distributed, generally substantially uniformly, throughout thematrix to produce a resiliently compressible material. Where the matrixis chosen from synthetic elastomers, one suitable method comprisesincorporating a polymer-based dilatant into a foamed syntheticelastomer. The dilatant may be incorporated during the formation of thefoam. For example, the foam-forming ingredients may be reacted to formthe foam in the presence of a solution or dispersion of the dilatant.Whatever method is used, however, while the dilatant may be incorporatedinto the pores of the foam, it is important that it does not completelydisplace the fluid from the pores.

The composite of the invention may include components other than thedilatant and fluid, e.g., fibrous and/or particular fillers,plasticisers, lubricants, extenders, pigments, dyes, etc. If desired,the composite of the invention may be incorporated within an envelopewhich may be rigid or flexible, but this is not essential. Likewise, itmay be associated with a rigid component but this is not essential forthe use of the composite and may even compromise some of its properties.

A coating may be applied to the composite, if desired.

The actual constitution of the composite will be influenced by theintended application. Applications cover a wide range of uses andinclude impact protection for objects, animals and humans. Potentialapplications extend to any dynamic situation where the object mayalready be in contact with a surface and the combination of object andsurface may undergo severe acceleration and/or deceleration, e.g. as inpackaging for delicate equipment or a human body in a vehicle seat.Thus, the nature of resiliently compressible mass, the amount of fluidin the mass, e.g. as indicated by the density of the mass, and thechoice and level of loading of the dilatant in the mass, will bedetermined by the requirements of the protective system in which thecomposite is to be employed. In general, the dilatant will form from 5to 80%, preferably 10 to 50%, more preferably 20 to 40% (such as 15 to35%) by volume of the composite, and the amount of fluid (in thepreferred case where it is gas) will be such that the fluid content ofthe composite is preferably about 30 to 90% (such as 20 to 90%) morepreferably about 45 to 90% (such as 30 to 80%) still more preferablyabout 55 to 85% (such as 40 to 70%) by volume. It should be noted thatthese proportions are excluding the use of any fillers or additionalcomponents.

The energy absorbing composite of the invention may be employed in awide variety of applications; for example in protective pads or clothingfor humans and animals, in or as energy absorbing zones in vehicles andother objects with which humans or animals may come into violentcontact, and in or as packaging for delicate objects or machinery.Specific examples of applications are in headwear and helmets;protective clothing or padding for elbows, knees, hips and shins;general body protection, for example for use in environments whereflying or falling objects are a hazard, vehicle dashboards, suspensionbushes, upholstery and seating. Other potential uses are in garments orpadding to protect parts of the body used to strike an object e.g. in asport or pastime; for example in running shoe soles, football boots,boxing gloves and gloves used in the playing of fives. This list is notintended to be exclusive and other potential uses will occur to thereader.

The following Examples illustrate the invention in which dilatantmaterials were incorporated into a solid foamed synthetic polymer matrixduring its manufacture.

EXAMPLE 1

This example details the inclusion of the pure polyborondimethylsiloxane(PBDMS) dilatant during the manufacture of polyurethane (PU) foam.

The base PU system is marketed by Jacobson Chemicals Ltd., Farnham,Surrey. The product is a modelling foam reference J-Foam 7087. This is atwo part system which requires the mixing of two components, part A andpart B in the ratio of 3 to 1 respectively. This mix can then be castinto open or closed moulds to produce a shaped foam component. Duringthe reaction of parts A and B a gas (believed to include carbon dioxide)is evolved to produce a closed cell structure in a PU soft foam.

The PBDMS supplied by the Chemical Institute, Warsaw, Poland waspre-mixed with the J-Foam part A at room temperature in a polyethylenebeaker by hand with the aid of a wooded spatula for approximately 15minutes until the mix appear homogenous. Various ratios of PBDMS to partA were trialed and are detailed as follows:

-   -   Trial 1-15 g PBDMS+40 g part A    -   Trial 2-15 g PBDMS+30 g part A    -   Trial 3-39 g PBDMS+50 g part A

Each of the above pre-mixes was then mixed with part B using the samemixing method and maintaining the 3 to 1 ratio of part A to part Birrespective of the amount of PBDMS. This mixing time was typicallyaround 10 seconds. These 3 component mixtures were next cast into a flatbottomed open polyethylene container and allowed to expand freely toproduce the foams.

As the PBDMS has a very much higher viscosity than either part A or partB, increasing the proportion of PBDMS produced a reduction in density ofthe resulting foam. The increased viscosity (melt strength) of the 3component mix restricted the expansion of the mix during reaction andcure stage. To establish the effect of reducing the viscosity of thePBDMS/part A pre-mix by heating this pre-mix an additional batch oftrial 3 was made with the pre-mix being heated to 65 degrees Celsiusbefore mixing with part B. This sample was then cast immediately aftermixing in the same way as previous specimens, but with the mouldpre-heated to 65 degrees Celsius also. The resulting densities of foamwere produced:

-   -   Trial 1-400 kg/m³    -   Trial 2-500 kg/m³    -   Trial 3 (65° C. pre-heated)-380 kg/m³

The densities were measured simply by weighing the samples and measuringthe linear dimensions to establish the total volume and dividing this bythe weight of the samples.

EXAMPLE 2

The same technique as shown in Example 1 was applied to the manufactureof PU foam containing Dow Corning 3179 silicone dilatant. This dilatantis a filled PBDMS where the percentage of PBDMS is 65% by weight. Thisrenders 3179 stiffer and stronger than the pure PBDMS. As a result ofthe presence of these fillers 3179 would not mix with J-Foam part A evenwith the assistance of an electric food blender. Using the electricblender 50 g of 3179 was dissolved in 40 g isopropyl alcohol (IPA), assolvent, then mixed with approximately 100 g of J-Foam part A. Thisproduced a creamy emulsion. In order to minimize the amount of IPApresent during the subsequent reaction with J-Foam part B the blenderwas left switched on with the 3179 and IPA and J-Foam part A mixture ina fume cupboard to encourage evaporation of the IPA. This was left for 1hour. The evaporation of the IPA over this period of time caused the3179 dilatant to come out of the solution and to form solid globules ofdilatant in the mix. The procedure was therefore repeated but during theevaporation stage the blender was stopped at 10 minute intervals toobserve visually the nature of the mixture. After 40 minutes tinyparticles of 3179 dilatant in suspension were just detectable with thenaked eye and at this stage part B was introduced into the mixture byhand and cast into an open container as before and again maintaining the3 to 1 ratio of part A to part B. The resulting foam had a measureddensity of 290 kg/m³ with a large closed cell structure (cell diametersapproximately 0.7 to 1.2 mm).

In order to increase the density of this foam the procedure was repeatedwith the addition of 35 g of PBDMS during the blending of the 3179dilatant, IPA and J-Foam part A to increase the viscosity of the mix.The resulting foam was of much smaller cell size (cell diametersapproximately 0.1 to 0.4 mm) and of higher density—640 kg/m³.

1. A self-supporting energy absorbing composite comprising: i) a solidfoamed synthetic polymer matrix; ii) a polymer-based dilatant, differentfrom i) distributed through the matrix and incorporated therein duringmanufacture of i); and iii) a fluid distributed through the matrix, thecombination of matrix, dilatant and fluid being such that the compositeis resiliently compressible.
 2. An energy absorbing composite as claimedin claim 1 in which the matrix is elastic.
 3. An energy absorbingcomposite as claimed in claim 1 or claim 2 wherein the matrix isselected from synthetic elastomers.
 4. An energy absorbing composite asclaimed in claim 3 wherein the synthetic elastomer is selected fromelastomeric polyurethanes.
 5. An energy absorbing composite as claimedin any one of the preceding claims wherein the polymeric dilatant isselected from silicone polymers exhibiting dilatant properties.
 6. Anenergy absorbing composite as claimed in claim 5 wherein the siliconepolymer is selected from borated silicone polymers.
 7. An energyabsorbing composite as claimed in any one of the preceding claimswherein the fluid is a gas.
 8. An energy absorbing composite as claimedin any one of the preceding claims containing polymeric dilatant withinpores of the foam.
 9. A method of forming a self-supporting energyabsorbing composite, which method comprises combining: i) a solid foamedsynthetic polymer matrix; ii) a polymeric dilatant, different from i),and iii) a fluid whereby the dilatant is distributed throughout thematrix by incorporating the dilatant into the foamed synthetic polymerduring manufacture of the matrix and the fluid is distributedsubstantially uniformly throughout the matrix to produce a resilientlycompressible-material.
 10. A method as claimed in claim 9 wherein thematrix is as claimed in anyone of claims 2 to
 4. 11. A method as claimedin claim 9 or claim 10 wherein the polymeric dilatant is as claimed inclaim 5, claim 6 or claim
 8. 12. A method as claimed in any one ofclaims 9 to 11 wherein the fluid comprises a gas.
 13. A self-supportingenergy absorbing composite comprising: i) a solid, closed cell foammatrix; ii) a polymer-based dilatant, different from i), distributedthrough the matrix; and iii) a fluid distributed through the matrix, thecombination of matrix, dilatant and fluid being such that the compositeis resiliently compressible.
 14. An energy absorbing composite asclaimed in claim 13 wherein the matrix is a synthetic elastomer and/oris as claimed in claim 2 or claim 3 or claim 4 and/or the polymericdilatant is as claimed in claim 5 or claim 6 or claim 8 and/or the fluidcomprises a gas.
 15. An energy absorbing composite obtained by a methodas claimed in any one of claims 9 to
 12. 16. An impact protection systemincluding an energy absorbing composite as claimed in any one of claims1 to 8 or 13 to 15.